1. Field of the Invention
The present invention relates to a method of manufacturing a bead inductor and also relates to a bead inductor produced by such a method and constructed for use in noise control circuits and other electronic components.
2. Description of the Related Art
As a noise-controlling device, particularly a device for use with a microprocessor, for example, which is required to carry a large electric current therethrough, a bead inductor has been proposed. A bead inductor which is an experimental device and has not been publicly disclosed, is formed of a resin material or a rubber material including a powdered magnetic substance, such as ferrite powder, with a conductor coil embedded therein. In this bead inductor, a conductor coil is embedded into a resin material or a rubber material via injection molding, etc., to form a molded body which is cut off at both ends thereof to expose both ends of the embedded coil. Then, metal caps are connected thereto by conductive resin paste or spot welding so as to define external terminals.
FIGS. 5 and 6 are sectional views illustrating a method for manufacturing such an experimental bead inductor. Referring to FIGS. 5 and 6, a metallic mold for injection molding to manufacture the bead inductor includes an upper mold 1 and a lower mold 2. A cavity is formed in the upper mold 1 and defines a space for molding a resin member. In the lower mold 2, a pin 4 is provided so as to be disposed in the cavities 3 when the upper mold 1 and the lower mold 2 are mated with each other. The upper mold 1 has a gate 1a for supplying a melted resin material into the cavity 3.
In order to manufacture a molded body of the bead inductor using the metallic mold shown in FIG. 5, the pin 4 is inserted into a conductor coil, which is defined by a wound, coated metallic wire. The metallic wire may be a copper wire and is preferably coated with a polyester resin, etc., for insulation. Then, the melted resin including a powdery magnetic substance such as ferrite powder is injected into the cavity 3 via the gate 1a. As a result, the outer portion of the conductor coil having the pin 4 inserted therein is molded of the melted resin.
FIG. 6 is a sectional view showing the state of the outside portion of a coil 5 molded in this manner. After the step shown in FIG. 5, the pin 4 is removed and the same resin material used on the outside portion of the coil 5 is injected into the space produced by the removing the pin 4, so as to mold the inside of the coil 5 of the melted resin, so that the coil 5 is embedded in the resin.
FIG. 7 is a sectional view showing a molded body obtained in this manner. The molded body 7 is formed of a molded resin portion 6 with the conductor coil 5 embedded therein. In FIG. 7 and other drawings which will be described in relation thereto, illustration of the molded resin portion 6 disposed in the inside 5a of the conductor coil 5 is omitted. The inside 5a of the conductor coil 5 is filled with the same resin as the outside molded resin portion 6.
In FIG. 7, lines A--A and B--B show cutting lines. The molded body 7 is cut off along the cutting lines via a dicing saw, or other cutting device, so that connecting terminal portions of the conductor coil 5 are exposed at the ends of the molded body. Metallic caps are mounted onto the connecting terminal portions of the coil which are exposed by the cutting. The metallic caps and the connecting terminal portions of the conductor coil are electrically connected to each other via conductive resin paste, spot welding, or the like.
FIG. 11 is a side view showing a bead inductor with the metallic caps attached thereto in this manner, while FIG. 12 is a plan view thereof. As shown in FIGS. 11 and 12, the metallic caps 8 and 9, which define external terminals, are attached to both ends of the molded body 7. As described above, the metallic caps 8 and 9 are electrically connected to the connecting terminal portions of both ends of the conductor coil 5 within the molded body 7.
When such a bead inductor is produced by the above-described method, there has been a problem of a low degree of reliability in the electrical connection between the inside conductor coil and the external terminal. That is, when the molded body 7 shown in FIG. 7 is cut off along the cutting lines A--A and B--B, the inside conductor coil may not be cut off along the cutting lines in one plane, resulting in a low degree of reliability of electrical connection.
FIG. 8 is a sectional view showing a cut-away section of the conductor coil in this case. The cut-away plane 7a of the molded body 7 is a cut-away surface when the molded body 7 is cut off along the cutting line A--A shown in FIG. 7, while the end portion 5c of the conductor coil 5 is cut along the plane so as to be flush with the cut-away plane 7a. FIG. 9 is a cross-sectional view showing the end portion 5c, in which a wire material such as a copper wire disposed in an insulation coating is exposed.
In contrast, the portion which is cut along the cutting line B--B shown in FIG. 7 corresponds to the cut-away plane 7b shown in FIG. 8, and the conductor coil 5 is not cut along the cut-away plane 7b, so that the wire material of the conductor coil 5 is cut off in a torn-off state to form the end portion 5b shown in FIG. 8.
FIG. 10 is a side view showing the state of the end portion 5b shown in FIG. 8. Since the wire material of the conductor coil 5 is coated for insulation by a resin layer, the inside wire material portion is exposed at only the end portion 5b in the cut-away plane 7b as shown in FIG. 10, leaving other portions covered by the insulation coating. In this state, the end portion 5b having a small area should be electrically connected to the metal cap, resulting in a low degree reliability of the connection.